Focus Testing

Part 5 - Focus Accuracy Check (for AF lenses)

A common complaint seen on web photo forums is that cameras (and/or lenses) aren't achieving accurate
focus. Sometimes the focus point is in front of the intended subject and sometimes it's behind it. Some of these observations
may be due to technical problems with AF, but some are due to user error or unreasonable user expectations.

Of course no manufacturer of cameras or lenses claims that autofocus is
absolutely perfect. For example, Canon state that the spec for focus on
"consumer" bodies (such as the Digital Rebels and the EOS 20D/30D/40D) is that
it should be within the DOF (Depth of Field). On the "pro" bodies (EOS 1 series) focus spec for f2.8 and faster lenses (f4 on some models) is that it should be within 1/3 of the DOF. With slower
lenses the AF accuracy of the "pro"
bodies reverts to the same as that of "consumer" bodies which don't have the
high precision sensors.

So the bottom line is that focus should be within the DOF, or to put it another way,
the image should look sharp. If it doesn't look sharp, focus is not likely to be in spec.

Note also that both the camera and lens are involving in focusing. A focusing error can be the
result of either a camera or a lens calibration problem.

The Chart

I've written this article so that users can test their DSLR under a set of
"standard" conditions. The test chart which is shown at
reduced size below can be downloaded (as a Zipped JPEG) HERE

You can see two sets of parallel lines which are your focus indicators. The numbers 1,
2 and 3 represent 1cm, 2cm and 3cm distances when the chart is used at 45 degrees as
explained below. In the center is a single vertical line, and that's your focus point.

The Test

To use the chart you set it up at 45 degrees to the axis of the lens as shown below.The easiest
way to do this is to lay the chart flat on the floor, and put the camera on a tripod looking
down at it at and angle of 45 degrees. In
this configuration (at 45 degrees) the 1, 2 and 3cm marks are correct. They are actually
spaced at 1.41, 2.82 and 4.23cm from the focus line, but when viewed at 45 degrees these
distances are modified by the Cosine of the viewing angle (Cos 45 = 0.707).

Use is easy. Once you have everything setup you select your focus point (let's use the
center point) and make sure the focus zone includes only the single focus target
line. It's important to note that the area of the AF sensor may not be exactly the same
as the markings on the viewfinder screen, so make sure there's nothing near the AF zone
but the focus line. Then you take your shots. Take several and refocus each time. Try several manual
focus shots. Use the maximum aperture of your lens so as to get minimum DOF.

The Results

Below is an example cropped from a frame taken with a EOS 10D using 50mm f1.8 lens at
f1.8 and focused manually on the single vertical line. Focus distance was about 0.45m (I
have the Mark I version of this lens which has a focus scale, the newer Mark II version
does not).

This image displays accurate focus. The "1", "2" and
"3" characters are pretty much equally blurred both in front of and behind the
focus line. There's maybe a very slight bias towards the front, but on the next manual
focus shot you might see an equal bias towards the rear. There's nothing wrong with the
manual focus of this this camera or lens. The lines are spaced at 2mm intervals. 1, 2 and
3 represent 1cn. 2cm and 3cm distances in front of and behind the focus line.

The next shot shows the same view, but taken using autofocus.

Here you can see a slight bias in front of the focus line. The "1
cm" mark is sharper in front of the focus line than behind it, though the focus line
itself is still quite sharp. I'd estimate that focus is maybe 2mm in front if the focus
line, but the line itself is still within the "sharp" zone, maybe close to the
rear limit of the DOF. For a 50mm lens at f1.8 focused at a distance of 0.45mm the
calculated DOF is 5mm (+/- 2.5mm). So the focus isn't "perfect" but it's pretty good and within spec
for autofocus. I'd suspect that such small focus offsets are pretty common but nobody ever
notices them. Below is the same shot at f5.6 and the focus offset is undetectable to my
eyes due to the increased depth of field at f5.6.